BOSTON — The number of safety violations at U.S. nuclear power plants varies dramatically from region to region, pointing to inconsistent enforcement in an industry now operating mostly beyond its original 40-year licenses, according to a congressional study awaiting release.

Nuclear Regulatory Commission figures cited in the Government Accountability Office report show that while the West has the fewest reactors, it had the most lower-level violations from 2000 to 2012 — more than 2½ times the Southeast’s rate per reactor.

The Southeast, with the most reactors of the NRC’s four regions, had the fewest such violations, according to the report, a copy of which was obtained by The Associated Press.

The striking variations do not appear to reflect real differences in reactor performance. Instead, the report says, the differences suggest that regulators interpret rules and guidelines differently among regions, perhaps because lower-level violations get limited review.

The study also says that the NRC’s West region may enforce the rules more aggressively and that common corporate ownership of multiple plants may help bolster maintenance in the Southeast.

However, the reasons aren’t fully understood because the NRC has never fully studied them, the report says. Right now, its authors wrote, the “NRC cannot ensure that oversight efforts are objective and consistent.”

Told of the findings, safety critics said enforcement is too arbitrary and regulators may be missing violations. The nuclear industry has also voiced concern about the inconsistencies, the report said.

The analysis was written by the GAO, the investigative arm of Congress, at the request of four senators. Before the government shutdown, the report had been set for public release later this month.

Steven Kerekes, a spokesman for the industry group Nuclear Energy Institute, declined to comment pending release of the report. The NRC’s public affairs office had no comment, citing the government shutdown.

The GAO analysis focuses on lower-level safety violations known as “nonescalated.” They represent 98 percent of all violations identified by the NRC, which regulates safety at the country’s commercial reactors.

Lower-level violations are those considered to pose very low risk, such as improper upkeep of an electrical transformer or failure to analyze a problem with no impact on a system’s operation, such as the effect of a pipe break. Higher-level violations range from low to high safety significance, such as an improperly maintained electrical system that caused a fire and affected a plant’s ability to shut down safely.

The GAO’s analysis shows 3,225 of these violations from 2000 through the end of 2012 across 21 reactors in the West. By contrast, there were 1,885 such violations in the Southeast. Yet that region is home to 33 reactors — 12 more than in the West. The West registered 153.6 violations per reactor, while the Southeast saw just 57.1.

The Midwest, with 24 reactors, had 3,148 violations, for a rate of 131.2 per reactor. The 26-reactor Northeast also fared worse than the Southeast, with 2,518 violations, or 96.8 per reactor.

The Cooper nuclear station in Brownville, Neb., led all sites in lower-level violations per reactor with 363. The next four were Wolf Creek, in Burlington, Kan., with 266; Kewaunee, in Kewaunee, Wis., 256; Perry, in Perry, Ohio, 256; and River Bend, in St. Francisville, La., 240.

The GAO found less regional variation in higher-level safety violations.

The six plants with the most higher-level violations per reactor from 2000 to 2012 were Davis-Besse in Oak Harbor, Ohio, with 14; Cooper, 11; Kewaunee, nine; Perry, eight; Palisades, in Covert, Mich., eight; and Fort Calhoun, in Fort Calhoun, Neb., eight.

“I believe the oversight process is totally arbitrary,” said Paul Blanch, an engineer who once blew the whistle on problems from within the industry and later returned to work on safety. He also said the NRC isn’t providing consistent training to inspectors and regional staff. Blanch was made aware of the GAO findings by the AP.

The report also indicates that some regulators may be missing small problems or giving them short shrift, safety experts said. And they said little violations can pile up and interact with one another to create bigger risks.

“Any time you start tolerating minor problems, you’re just setting the stage for major safety problems down the road,” said nuclear engineer David Lochbaum of the Union of Concerned Scientists. He once trained NRC staff.

Phillip Musegaas, a lawyer with the environmental group Riverkeeper, said regulators should do more to make sure that lower-level violations are fixed. “NRC’s tracking to make sure these violations are resolved is completely ineffectual,” he said.

In its official response to the report, the NRC defended the objectivity of its plant assessments. At the same time, it acknowledged the regional differences and promised to look deeper into why they happen.

According to the GAO, the NRC regulatory staff also offered several explanations, including regional variations in reactor ages and time spent on inspections. However, the congressional watchdog said those explanations are not supported by the data.

The agency did offer that regulators may be right when they cite the possible impact of joint ownership of sites, a circumstance most prevalent in the Southeast. Nuclear plants under one owner may benefit from more corporate resources and thus avoid violations, the GAO suggested. The NRC also said higher-level violations are more consistent because they are more deeply reviewed.

On other issues, the report said the NRC needs an easier-to-use internal database to help staff learn of historical safety issues that apply to current problems. It also pressed the agency to improve its public documents website to allow tracking of safety violations. The NRC said it would work to improve both tools.

Senators requested the report in reaction to the Japanese nuclear accident at Fukushima in 2011 and an Associated Press investigative series later that year about aging U.S. nuclear plants. The AP series found that federal regulators had relaxed safety rules to keep plants running beyond their initial licenses. It also reported leaks of radioactive tritium at three-quarters of plants and outdated estimates of evacuation times.

The GAO analyzed data for 104 commercial reactors, but four permanently shut earlier this year: Crystal River in Florida, Kewaunee, and the two units at San Onofre in California.